A number of studies are underway which are designed to improve our understanding of the dynamics of the Ames Salmonella microsome test, to make improvements in the standard protocols, to make the test easier to run, and/or to make the test results more readily interpretable. These studies include the effects of substances which may be present in environmental mixtures and which will interfere with a positive mutagenic response in the apparent absence of cell toxicity; characterization of the new tester strains, TA97 and TA102; the effect of prolonged storage of mutagen solutions used for positive controls; and the effect of the commonly-used solvent, DMSO, on the Salmonella tester strains.